Coupling optical fibers to a photonic integrated circuit (PIC) chip is a difficult task, especially for multiple fiber connections at once. This is because the size of the optical field inside the PIC waveguides is on the order of a micron, which means that the fiber connection needs to be positioned with sub-micron precision in three dimensions to achieve an optimal coupling. For attachments of one or two fibers to a single PIC output/input waveguide, there are standard methods, which are undesirably costly. However, no good, reliable and cheap solutions exist to attach multiple fibers to a PIC chip.
Multiple fiber-chip couplings have been carried out so far by using a lensed fiber array that is accurately aligned to the outputs of the PIC chip and then fixed and possibly fine-adjusted after fixing. This is difficult and costly and the precision with which fiber arrays are fabricated is on the order of a micron, and reaching sub-micron precision requires careful selection of individual fibers. Another method uses a lens-array for beam expansion. Careful alignment of a second lens-array is needed. Both methods require “active alignment” where light is generated or detected on the PIC chip and the optical coupling is optimized by maximizing the coupled optical power to the optical fiber arrays. Passive alignment is a simpler process where parts are aligned without illuminating them and optimizing optical signals.
It would be an advance in the art to provide high-performance optical coupling to a PIC chip using passive alignment.